This invention relates to the field of microwave radio frequency waveguide devices and to the conversion between selected transverse magnetic and traverse electric operating modes in such waveguide.
In the design of radio frequency apparatus which operates in the ultra high and microwave range of frequencies it is often necessary to consider configurations of the electric and magnetic fields which exist inside waveguide elements communicating signals between portions of the apparatus. The configuration of these electric and magnetic fields is determined by boundary conditions imposed by the waveguide walls and by the constraints of Maxwell's equations which require that there be no tangential components of electric field and no normal component of magnetic field existing at the waveguide walls. Usually a plurality of independent field configurations can meet these constraints in a particular waveguide disposal. Each of these field configurations is referred to as a waveguide operating mode or more simply as a mode. Knowledge of these modes in a radio frequency energy generating and utilizing apparatus is often essential in order to achieve any functional operation or efficient operation of the apparatus. In the virtual cathode microwave oscillator (VCO), for example, electromagnetic energy is usually generated in one or more TM.sub.0n modes in a circular waveguide, however, utilization of this energy in a conical antenna element is inconvenient or impractical primarily because of the null on axis antenna pattern which results from feeding an antenna with energy in this mode. Conversion of TM.sub.0n mode energy to the TE.sub.11 mode is a desirable addition to this combination since a peaked on axis antenna beam can thereby be achieved.
Background discussions of waveguide modes are to be found in the textbooks "Antenna Engineering" authored by R. C. Johnson and H. Jasik 2nd edition, McGraw-Hill Incorporated, 1984 especially in chapter 42, and in "Electronic and Radio Engineering" authored by Frederic E. Terman, McGraw-Hill Incorporated, 1955 especially in chapter 5 and in "Principles and Applications of Waveguide Transmission" authored by George C. Southworth, D. Van Nostrand Company Incorporated, 1950 all of which are hereby incorporated by reference herein. The footnotes to chapter 5 in the Terman textbook identifies some of the earliest work in the use of waveguides and transmission systems for very high frequency electrical signals. Each of these textbooks illustrate the magnetic and electric field patterns comprising the TM.sub.01 and TE.sub.11 circular waveguide modes.
In topics of present technical endeavor, both the performance of High Power Microwave (HPM) susceptibility testing and the development of compact HPM sources for use as directed energy weapons require the availability of HPM sources and systems that are compatible with antennas in order to produce radiation in a forward-directed centrally-peaked pattern. Some of the highest peak power HPM sources reported to date (e.g., VCOs and related sources) however, produce microwave energy in the TM.sub.01 mode in circular waveguide. The production of microwave energy in the TM.sub.01 mode is, in fact, a fundamental consequence of the physical processes by which such devices operate.
As a result of these and other needs in the art there have been a number of methods employed to convert microwave energy in the TM.sub.01 mode to the TE.sub.11 mode, however each of these methods have several disadvantages, and thus have not found widespread application. Earlier TM.sub.01 to TE.sub.11 mode conversion methods include, for example:
1. Simple waveguide bends: a typical waveguide bend-type converter is described in U.S. Air Force Weapons Laboratory Technical Report AFWL-TR-88-31 "Low-Frequency High-Power Microwave Source Development" which was authored by M. D. Haworth et al, February 1989. Copies of this report may be obtained by selected persons from the Commander of the U.S. Air Force Weapons Laboratory, Kirtland Air Force Base, New Mexico. Although this converter is capable of broadband operation at very high power levels, it cannot exceed a theoretical maximum of 62.5% efficiency, is bulky and heavy, difficult to machine, and produces radiation at an inconvenient angle to the source axis.
2. Serpentine waveguide bends: A serpentine waveguide bend-type converter is described by M. J. Buckley, G. H. Luo, and R. J. Vernon, in "New Compact High-Efficiency Mode Converters for High Power Microwave Tubes with TE.sub.0n or TM.sub.0n Mode Outputs," 1988 IEEE MTT-S Digest, p 797-800. The serpentine waveguide design for a TM.sub.01 to TE.sub.11 mode converter described in this report is, in fact, capable of operation at very high power levels and at extremely high efficiency (98%), but is bulky and heavy, very difficult to machine, and is only efficient over a narrow frequency band.
3. Rotary joint converters: A waveguide rotary joint converter is described by G. L. Ragan, in "Microwave Transmission Circuits" in Volume 9 of the Radiation Laboratory Series, McGraw Hill 1948. This converter is actually a rectangular TE.sub.10 to TM.sub.01 and back to rectangular TE.sub.10 converter originally designed to allow two rectangular waveguides to be joined in a rotatable joint. By use of only half of the joint, one may convert a circular TM.sub.01 mode to a rectangular TE.sub.10 mode. Microwave power in the rectangular TE.sub.10 mode may be radiated as an on-axis pencil beam from a pyramidal horn, in principle achieving the same advantages as conversion to the circular TE.sub.11 mode. The rotary joint converter is not generally used with HPM sources, however, since it utilizes a resonantly tuned shorting plate and is therefore highly efficient only over a very narrow frequency band.
Prior patents have also considered the subject of microwave mode conversion and other uses of apparatus having structural similarities to mode converter devices. The U.S. Pat. No. of J. Barker, 2,816,271, for example, discloses a microwave mode converter in which a gas filled thin fin member is disposed in a circular waveguide in order that the waveguide have altered characteristics at differing levels of input energy. Additionally U.S. Pat. No. 3,896,449 issued A. E. Blume, describes a horn antenna arrangement which includes higher order mode compensating provisions including the use of flared or conical frustum waveguide sections.
U.S. Pat. No. 3,955,202, issued to P. T. K. Young, additionally shows the use of tapered wedge members within a circular or rectangular waveguide in order to generate the desired polarization of signals entering a horn antenna element. Additionally the U.S. Pat. No. 4,510,469 of D. F. Bowman discloses the use of shaped dielectric members in combination with tapered waveguide sections in order to achieve mode conversion. Additional background information somewhat relevant to the present invention is to be found in a number of published textbook references including the above mentioned Radiation Laboratory Series of texts published by McGraw Hill Book Company particularly in chapter 6 of volume 9 and especially in section 6.23 of this chapter 6. Additionally the text "Field and Wave Electrodynamics" by C. C. Johnson also published by McGraw Hill in 1965 and particularly sections 4.12-4.17 therein provide interesting background and somewhat relevant prior art with regard to the present invention.
The subject of broadband high efficiency mode converters is also treated in the article "New Compact Broadband High-Efficiency Mode Converter for High Power Microwave Tubes With TE.sub.0n or TM.sub.0n Mode Outputs" authored by M. J. Buckley et al which appears in the 1988 Institute of Electrical and Electronic Engineers microwave theory digest at page 797.
Although each of these examples of prior work in the field of mode converters is of interest with respect to the present invention, none of these efforts have achieved the benefits of a compact broadband high power circular TM.sub.01 to TE.sub.11 waveguide mode converter nor used the combination of waveguide structure disclosed in the present invention.